It is known to surgically implant a medical device in a patient's body to achieve a number of beneficial results. In order to operate properly within the patient, a reliable, consistent power link between the medical implant and an external control module is often necessary to monitor the implant's performance or certain patient parameters and/or to command certain operations by the implant. This power link has traditionally been achieved with TET systems that communicate across a small amount of tissue, such as relatively thin dermal tissue across the front of the patient's shoulder, such as for cardiac pacemakers.
In some instances, multiple coils have been suggested in regard to a TET or telemetry system in order to provide additional flexibility in aligning primary and secondary coils. For instance, U.S. Pat. No. 6,058,330 Borza discloses a transcutaneous system in which multiple coils were used in the secondary circuitry and perhaps also in the primary circuitry. However, in this instance secondary coils are spaced about the patient's body for the purpose of mitigating tissue damage due to long term exposure to strong electromagnetic fields in a continuous application wherein the medical implant is continuously TET powered or continuously engaged in telemetry. Thus, the '330 Borza patent teaches combining the power received from multiple secondary coils that are widely spaced, either a selected one of the secondary coils is receiving a strong electromagnetic signal or multiple secondary coils are simultaneously receiving weaker electromagnetic signals so that the dermal tissue overlying any one secondary coil is not continuously exposed to a strong electromagnetic signal.
Another problem with continuously coupling electromagnetic power to a secondary coil is the inconvenience to the patient of having the primary coil externally fixed in place, hampering movement and causing discomfort. U.S. Pat. No. 6,366,817 to Kung discloses using multiple coils in the primary spaced about the patient with circuitry that detects which primary coil is best oriented to efficiently couple electromagnetic energy to the implanted secondary coil and thus switching current to the selected primary coil.
U.S. Pat. No. 6,463,329 to Goedeke discloses multiple primary telemetry coils whose major surface, defined by their exterior, are parallel to one another and spaced apart. These coils are used to initiate telemetric communication between the programmer or monitor and the implanted device. At the frequencies disclosed, these coils are employed as loop antennas rather than inductively coupled coils. Since the antenna pattern of a loop antenna includes a “null” when very close to the loop, this approach is used to switch between primary coils when necessary to communicate with the secondary coil, thus primarily addressing problems with medical implants placed under a thin layer of dermal tissue that could coincide with the null of a primary coil placed in contact with the patient.
U.S. Pat. No. 5,741,316 to Chen et al. discloses a transmitter coil that has half of the windings on one leg of a horseshoe magnetic conductor in series with another half of the windings on the other leg. The magnetic flux contribution of each winding portion is thereby combined. However, the corresponding requirement for a horseshoe-shaped magnetic core in the implanted device is undesirable due to the increased size. Thus, laterally offset, electrically serial windings would not be a benefit for a secondary coil integral to an implanted device that lacks a horseshoe shaped magnetic conductor.
While these approaches to improving the effectiveness of electromagnetic coupling to a medical implant have merit, we have recognized an application that does not benefit from spaced apart multiple primary coils and/or spaced apart secondary coils, yet a need exists for enhanced power transfer efficiency. An implantable medical device that may benefit from use of enhanced TET is an artificial sphincter, in particular an adjustable gastric band that contains a hollow elastomeric balloon with fixed end points encircling a patient's stomach just inferior to the esophago-gastric junction. These balloons can expand and contract through the introduction of saline solution into the balloon. In generally known adjustable gastric bands, this saline solution must be injected into a subcutaneous port with a syringe needle to reach the port located below the skin surface. The port communicates hydraulically with the band via a catheter. While effective, it is desirable to avoid having to adjust the fluid volume with a syringe needle since an increased risk of infection may result, as well as inconvenience and discomfort to the patient.
To that end, we have recently developed implanted infuser devices that regulate the flow of saline without requiring injection into the subcutaneous port. This system transfers AC magnetic flux energy from an external primary coil to a secondary coil that powers the pump in the implant connected to the gastric band within the abdomen. Although batteries may be used to power the device, these long-term devices benefit from use of TET, allowing an implanted device of reduced size and complexity. Moreover, these devices may remain unpowered between adjustments, which provides additional advantages. These implantable, bi-directional infusing devices that would benefit from enhanced TET powering and/or telemetry are disclosed in four co-pending and co-owned patent applications filed on May 28, 2004, the disclosure of which are hereby incorporated by reference in their entirety, entitled (1) “PIEZO ELECTRICALLY DRIVEN BELLOWS INFUSER FOR HYDRAULICALLY CONTROLLING AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Ser. No. 10/857,762; (2) “METAL BELLOWS POSITION FEED BACK FOR HYDRAULIC CONTROL OF AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Rocco Crivelli, Ser. No. 10/856,971; (3) “THERMODYNAMICALLY DRIVEN REVERSIBLE INFUSER PUMP FOR USE AS A REMOTELY CONTROLLED GASTRIC BAND” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Ser. No. 10/857,315; and (4) “BI-DIRECTIONAL INFUSER PUMP WITH VOLUME BRAKING FOR HYDRAULICALLY CONTROLLING AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Ser. No. 10/857,763.
Unlike the previously mentioned medical implants, an infuser device for an artificial sphincter is typically implanted below a thicker dermal layer of skin and adipose tissue. This is particularly true for patients that typically receive an adjustable gastric band as a treatment for morbid obesity. Moreover, being more deeply implanted may allow for greater client comfort. However, the thickness of tissue presents difficulties for effective power coupling from a primary TET coil.
It is desirable that the secondary coil be encompassed within an outer case of the infuser device to enhance the integrity of the device. It is especially desirable to not have one or more secondary coils detached from the infuser device and implanted more superficially, as this complicates the implantation and explantation of the infuser device. Consequently, these generally known approaches to having spaced apart secondary coils to additively contribute to received signals are not appropriate. Further, there are physical and electromagnetic constraints to configuration of a secondary coil that is encompassed within a medical implant, especially the diameter, the number of turns of the coil, and the diameter of each turn.
Consequently, in order to provide for a larger power transfer range between the primary and secondary TET coils a significant need exists for enhancing power coupling with a deeply implanted medical device within the dimensional constraints imposed upon a secondary coil.